OK, I still think that there should be some difference for characters going in mission and other "staying home". Not talking necessarily about more or less XP, could be some other form of reward. Maybe those that stay home regain full HP/PP (or there could be some other variable like injured body parts?) but get less XP from the mission (still get a share, but inferior).
Anyway stuff that probably will be more clear once the testing begins

Why? What are you trying to accomplish with the differentiation?
There might also be a problem from the writers end, if not all members are part of mission. More then with me, you should probably talk with Shaun about that part.